The Democrats will keep control of the US Senate after a projected win in the state of Nevada.
Democrat incumbent Catherine Cortez Masto is sitting on 48.7% of the vote - a narrow edge over Republican former state attorney general Adam Laxalt on 48.2%.
Ms Cortez Masto, 58, campaigned heavily on the issue of abortion and also criticised Mr Laxalt over his ties to major oil companies, which have enjoyed record profits in recent months.
Mr Laxalt had tried to tie Ms Cortez Masto to President Joe Biden's economic policies, blaming them both for inflation and rising fuel prices.
The election result comes a day after Democrat Mark Kelly claimed victory in Arizona, beating his Republican challenger Blake Masters, who - like Mr Laxalt - had been endorsed by former president Donald Trump.
This means the Democrats have 50 seats in the Senate.
Democrat vice president Kamala Harris can break ties in the 100-member chamber, meaning that the party can secure victories for President Biden.
This will be particularly important if a seat on the US Supreme Court - which currently has a 6-3 conservative majority - were to open up in the final two years of Mr Biden's term.
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Speaking to journalists in Cambodia ahead of an East Asia Summit, President Biden said he was "incredibly pleased" with the election results.
He said: "We're focusing now on Georgia. We feel good about where we are. And I know I'm a cockeyed optimist. I understand that.
"Again, I'm not surprised by the turnout. I'm incredibly pleased. And I think it's a reflection of the quality of our candidates."
He added: "I feel good. I'm looking forward to the next couple of years."
Senate Majority Leader Chuck Schumer said: "We got a lot done and we'll do a lot more for the American people.
"The American people rejected - soundly rejected - the anti-democratic, authoritarian, nasty and divisive direction the MAGA Republicans wanted to take our country."
The final state to decide is Georgia, where Democratic senator Raphael Warnock faces a run-off against Republican Herschel Walker on 6 December.
The 435-seat House of Representatives remains in the balance, however.
The Republicans have the edge but votes are still being counted in several races, including many in liberal-leaning California.
If Democrats manage to pull off a win in the House, it would mean full control of Congress for Democrats - and another chance to advance Mr Biden's priorities.
The party still lacks the 60 votes in the Senate needed to move many kinds of major legislative changes.